It has been known for a long time to change the characteristics of polyethylene and other olefinic polymers by heating such polymers in the presence of cross-linking agents such as organic peroxides. Disadvantageously, however, this process requires particularly careful process controls to prevent premature or too strong combination of the polymer molecules which results in formation of a solidified polymer mass in the apparatus. Also, the objects made from such modified polymers typically are not shape retentive.
Therefore, efforts have been made to devise a better controllable and adjustable process for the cross-linking of olefinic polymers. One such a process consists of grafting an organosilane compound onto the polymer. Such a process has entered the literature as Sioplas-process (cf. Wire Journal, May 1977, pp. 88-92, "Crosslinked Polyethylene Insulations Using the Sioplas Technology"). The organosilane compound contains at least one silicon-bound, hydrolyzable group and one group capable of reacting with free radical sites, which subsequently results in a cross-linking by the influence of moisture by way of the silane, coupling reaction.
There are several known proposals to conduct such a process. In the British Pat. No. 1,234,034 (Midland Silicones), the grafting takes place by mechanically processing the mixture in which case free radical sites develop on the polymer which may be joined by the silane compound. British Pat. No. 1,286,460 (Dow Corning) describes adding compounds which form free radicals, which compounds have a half-life period of less than 6 minutes at the given reaction temperatures; a temperature of about 140.degree. C. is proposed as a reaction temperature. The free radical initiators are organic peroxides and per-esters such as, for example, benzoyl peroxide, dicumyl peroxide, azo compounds and others.
The process may also be applied to certain cross-linked or cross-linkable copolymers of ethylene and alkylracrylates as is described in British Pat. No. 1,396,120 (Dow Corning).
In the course of further research in this field, it turned out that under certain circumstances, the first stage of the above-cited process is critical insofar as the melting index of the polymers grafted with the silane compound in the presence of peroxides and peresters was clearly reduced in relation to the starting polymers. Whenever now the starting material had already a relatively low melting index, then it could occur that in the case of a subsequent shaping as a result of the melting index which was reduced still further, problems resulted. The selection of very special free radical builders (initiators) for carrying out the process in such a case was capable of preventing this difficulty. The suitable free radical builders turned out compounds having the general formula: ##STR1## wherein R" is a secondary or tertiary alkyl radical, a cycloalkyl radical having greater than 4 carbon atoms or a benzyl radical, and R"' is a tertiary alkyl radical or a cumyl radical. Here too, the reaction is carried out again at a temperature at which the free radical builder has a half-life period of below 6 min. This process is described in the British Pat. No. 1,450,934 (Dow Corning) and in the French Pat. No. 2,217,363 (Dow Corning) based on the former.
Basically, one conducts, in that case, the mixing and grafting either with cascade extruders, such as according to the Austrian Pat. No. 362,569 (BICC) or ther German OS No. 2 636 709 (Gutehoffnungshuette) or with a very long, one-worm extruder according to the Austrian Pat. No. 345,554 (BICC-Maillefer). The path by way of the cascade extruder is very expensive and is connected with high investments as a result of which the profitability is not questioned to be sure, but the write-off of such an installation must be made over a prolonged period of time. Far more interesting is the process by way of the one-worm extruder.
The process using the one-worm extruder is described in the Austrian Pat. No. 345,554. A process for producing a cross-linked, extruded product from a graftable and cross-linkable polymer with the help of a hydrolyzable, unsaturated silane compounding agent, includes mixing the polymer and the entire compounding agents in the same cylinder of a worm extruding machine. The worm extruding machine has, in sequence, a premixing zone, a homogenizing zone and is a dosing zone.
At the same time, as results from a reference to the British Pat. No. 964,428, the homogenizing zone is either formed by a worm having a decreasing inclination in the direction toward the extruding head wherein the base of the groove remaining between the bridge in the area of the second half of the homogenizing zone is provided with bores leading to a central channel of the worm which leads to the end of the worm facing the extruder head, or which is formed by a two-thread worm whereby the two threads have a variable rise so that the two threads pass over into one another at the beginning and at the end of the homogenizing zone. The molten material is transferred from the one spiral groove into the other spiral groove by way of the cylinder between the bridges of the worm and the cylinder surrounding them. In practice, the bridges of the worm showing a variable inclination act as dynanic mixer. However, such worms can be produced only at very great cost.